Mobile terminals for communication such as wireless devices are also known as e.g. User Equipments (UE), wireless terminals and/or mobile stations. Mobile terminals are enabled to communicate wirelessly in a wireless communications system or cellular communication network, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two mobile terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Mobile terminals may further be referred to as mobile telephones, cellular telephones, laptops, or tablet computers with wireless capability, just to mention some further examples. The mobile terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The wireless communication system covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, Base Transceiver Station (BTS), or Access Point (AP), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for mobile terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
The 3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
In a wireless communication system, when a mobile terminal is connected to a base station, it is important, from a performance point of view, that the mobile terminal is connected to the most suitable base station. Often the most suitable base station is the one from which the mobile terminal receives the best signal quality and/or strongest received signal strength. Better signal quality and/or strongest received signal strength in general means that a higher data rate can be supported, that the delay is smaller, and/or in general that the relevant Quality of Service (QoS) is better. When the wireless communication system is heavily loaded, it may not be enough to only consider the Signal quality and/or received signal strength as there simply will not be sufficient available resources. These resources may either be in an access link between the mobile terminal and the base station, and/or it may be in the wireless communication system connecting different base stations to each other.
In practise, to ensure that the mobile terminal is connected to the most suitable base station, the mobile terminal may scan for signals from different base stations within radio range and then try to connect to most suitable base station. Here, most suitable base station may for instance be the base station that is expected to provide the best QoS for the mobile terminal.
As the mobile terminal moves, it may find that the base station it is presently connected to is no longer the most suitable, and therefore the mobile terminal may be handed over from one base station to another. This Handover (HO) procedure target is that the mobile terminal is always connected to the most suitable base station. However, the HO procedure comes at a cost as the mobile terminal needs to perform measurements, which are of no use in case a HO will not be performed, and it also takes resources from the wireless communication system to perform the handover.
In addition, for certain applications with stringent requirements a HO may be hard to perform without causing a glitch or interruption in the communication. For this reason, it may actually be preferred that the mobile terminal remains connected to one base station for a longer time than to continuously perform handovers between different base stations just to ensure that the mobile terminal is connected to the very best one. Specifically, in case the mobile terminal is moving fast and the coverage area of the base station is small it is not a good solution to connect to another base station, even if the signal quality would be better, as the time duration between HOs in this case may be expected to be rather small. To address this problem, the wireless communication system should preferably have at least some cells that are large, i.e., macro cells, which fast moving mobile terminals may connect to, whereas slowly moving or stationary mobile terminals may connect to smaller cells, for instance micro- or pico cells. In case the mobile terminal is in idle mode and performs cell reselection similar aspects apply.
Today, some mobile terminals support several Radio Access Technologies (RAT), and in particular cellular access standards such as WCDMA and LTE as well as Wireless Local Area Network (WLAN) access are supported. This means that when the mobile terminal is scanning for being connected to the most suitable base station it may be even more challenging as the base station may either be a WLAN Access Point (AP) or a base station in a cellular communications network. Typically the WLAN AP will have a rather limited coverage area, and therefore it is not suitable for the mobile terminal to connect to the WLAN AP if the mobile terminal is moving fast. In fact, in order to save power, the mobile terminal may determine not to scan for WLAN APs when the mobile terminal is moving fast because it may then result in two HOs, i.e. one to and one from the WLAN AP, and the mobile terminal is only able to stay connected to the WLAN AP for a very short time.
As mentioned above, to ensure that the mobile terminal is connected to the most suitable base station, the mobile terminal may scan for signals from different base stations within radio range trying to connect to the most suitable one. However, as scanning consumes power, scanning for small cells such as e.g. WLAN APs are typically avoided unless necessary. In particular this may be the preferred setting in case it is known that the mobile terminal is moving at relatively high speed.
A problem is that when also the base stations are moving in the wireless communication system it is difficult to decide which mobile terminal that shall connect to which base station.